JPWO2012120732A1 - Optical pumping magnetometer, magnetoencephalograph and MRI apparatus - Google Patents

Abstract

The optical pumping magnetometer is an optical pumping magnetometer that measures a magnetic field to be measured using optical pumping, and includes a non-magnetic cell having at least an alkali metal sealed and having optical transparency and heat resistance, and laser light. A laser beam irradiation unit that optically pumps at least the alkali metal, a detection unit that receives a transmitted laser beam transmitted through the cell and detects a detection signal related to the magnetic field, and the cell. A high frequency voltage application unit that applies a high frequency voltage to the pair of application units and heats the cell by dielectric heating.

Description

  The present invention relates to an optical pumping magnetometer, a magnetoencephalograph, and an MRI apparatus.

  In recent years, an optical pumping magnetometer using an alkali metal has been developed as a high-sensitivity magnetometer that replaces the SQUID sensor. The optical pumping magnetometer does not require a cooling function and can greatly reduce the running cost of the SQUID sensor. Therefore, the optical pumping magnetometer is expected to be applied to a magnetoencephalograph, an MRI apparatus, and the like.

  In such an optical pumping magnetometer, a laser beam is irradiated to a cell in which an alkali metal is enclosed, whereby the alkali metal in the cell is spin-polarized. And the detection signal regarding the magnetic field of measurement object is detected by receiving the transmitted laser beam which permeate | transmitted the cell in this state. Here, when spin-polarizing an alkali metal, it is necessary to heat the cell to a predetermined temperature or higher in order to vaporize the alkali metal. For example, as described in Patent Document 1 below, A cell may be heated by installing a heater directly and operating this heater. In some cases, a hot air generator is provided instead of the heater, and the cell is heated by blowing hot air from the hot air generator.

JP 2009-10547 A

  However, when the cell is heated by the heater as described above, it is difficult to install the heater at a location where the transmitted laser beam is transmitted through the cell, and thus the cell may not be heated uniformly. Sensitivity may deteriorate. On the other hand, when the cell is heated by the hot air generator as described above, the hot air generator is separately required, so that the magnetometer may be increased in size and complicated. Further, in this case, since there is a possibility that noise is generated from the hot air generator, a configuration for removing the noise is required.

  Then, this invention makes it a subject to provide the optical pumping magnetometer, the magnetoencephalograph, and MRI apparatus which can heat a cell simply and uniformly.

  In order to solve the above-described problem, an optical pumping magnetometer according to one aspect of the present invention is an optical pumping magnetometer that measures a magnetic field to be measured using optical pumping, and includes at least an alkali metal sealed and transmits light. Non-magnetic cell with heat resistance and heat resistance, a laser beam irradiation unit that irradiates the cell with laser light and optically pumps at least alkali metal, and a transmitted laser beam transmitted through the cell, and detects a detection signal related to the magnetic field And a high-frequency voltage application unit that applies a high-frequency voltage to a pair of application units provided in the cell and heats the cell by dielectric heating.

  In this optical pumping magnetometer, the cell is heated by dielectric heating by applying a high-frequency voltage to the cell through the application unit by the high-frequency voltage application unit, that is, the cell itself generates heat by the dielectric loss of the cell, and the cell Can be heated. Therefore, the cell can be easily and uniformly heated.

  In addition, the pair of application units is electrically connected to one outer surface of the cell so as to extend over the entire region of the one outer surface, and to the other outer surface facing the one outer surface of the cell, And a second electrode electrically connected so as to extend over the entire outer surface. In this case, the cells can be heated evenly with higher accuracy.

  Moreover, a pair of application part may have a light transmittance. Thereby, even when a pair of application parts are provided on the optical path of the transmitted laser light, it is possible to suppress deterioration in accuracy of magnetic field measurement.

  Moreover, the control part which controls operation | movement of a detection part and a high frequency voltage application part is provided, and a control part may stop the application of the high frequency voltage by a high frequency voltage application part, when a detection signal is detected by a detection part. In this case, it is possible to suppress the adverse effect on the magnetic field measurement due to the magnetic field generated by the operation of the high frequency voltage application unit.

  Moreover, you may provide the external magnetic field shielding part which covers a measurement object and a cell so that an external magnetic field may be shielded. In this case, it is possible to suppress the adverse effect of the external magnetic field on the magnetic field measurement.

  Further, as a configuration that preferably exhibits the above-described effects, specifically, the detection unit receives the laser beam irradiated by the laser beam irradiation unit and transmitted through the cell as a transmitted laser beam, and the laser beam before and after transmission through the cell is received. A detection signal is detected based on the change in light intensity, or linearly polarized laser light that has been irradiated by a laser light irradiation unit other than the laser light irradiation unit and transmitted through the cell is received as transmitted laser light and transmitted through the cell. There is a configuration in which a detection signal is detected based on a change in polarization state of the linearly polarized laser beam before and after.

  A magnetoencephalograph according to one aspect of the present invention includes the optical pumping magnetometer. Also in the magnetoencephalograph of the present invention, the above-mentioned effect of simply and uniformly heating the cell is exhibited.

  An MRI apparatus according to one aspect of the present invention includes the optical pumping magnetometer. In the MRI apparatus of the present invention as well, the above-described working school effect of simply and uniformly heating the cell is achieved.

  According to the present invention, the cells can be easily and uniformly heated.

It is a schematic block diagram which shows the optical pumping magnetometer which concerns on one Embodiment of this invention. It is a schematic perspective view which shows the glass cell in the optical pumping magnetometer of FIG. It is a schematic perspective view which shows the glass cell in the modification of the optical pumping magnetometer of FIG.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the following description, the same or equivalent elements will be denoted by the same reference numerals, and redundant description will be omitted.

  FIG. 1 is a schematic configuration diagram showing an optical pumping magnetometer according to an embodiment of the present invention, and FIG. 2 is a schematic perspective view showing a glass cell in the optical pumping magnetometer of FIG. As shown in FIGS. 1 and 2, the optical pumping magnetometer 1 of this embodiment measures the magnetic field of a weak magnetic field source (measurement target) P using optical pumping (spin polarization). It is used as a highly sensitive magnetometer in magnetoencephalographs and MRI (Magnetic Resonance Imaging) devices.

  A magnetoencephalograph is a device that measures an active site in the brain, a function (function) of the brain, and the like by detecting a very weak magnetic field generated from the brain accompanying brain activity from the outside of the brain. The MRI apparatus is an apparatus that captures an image of a body in a non-contact manner by detecting a spatial distribution state of hydrogen nuclei (protons) that occupy most of a human body constituent material using a nuclear magnetic resonance phenomenon. This MRI apparatus is also called a nuclear magnetic resonance imaging apparatus.

  The optical pumping magnetometer 1 includes a glass cell (cell) 2, a laser light source 3, a detector (detection unit) 4, a high frequency power source (high frequency voltage application unit) 5, and a controller 6.

  The glass cell 2 is a hollow body having an internal space 2a and has, for example, a cubic outer shape having a thickness of 2 to 3 mm, a length of 40 mm, a width of 40 mm, and a height of 40 mm. The glass cell 2 is made of heat-resistant glass, so that the glass cell 2 is non-magnetic having light transmittance and heat resistance. In the internal space 2a of the glass cell 2, at least an alkali metal 10 and a buffer gas 11 are sealed so as to be airtight with respect to the outside.

  Examples of the alkali metal 10 include potassium, rubidium and cesium. Here, rubidium is preferably used for the alkali metal 10. The alkali metal 10 is solid or semi-solid at normal temperature, and is vaporized at a predetermined temperature (for example, about 200 ° C.) or higher. As the buffer gas 11, a rare gas such as helium is used. A quench gas such as nitrogen may be sealed in the internal space 2a of the glass cell 2.

  The glass cell 2 is covered with a heat insulating material D, and is configured so that the heat is not radiated to the outside. In the heat insulating material D, an opening Da is formed on the optical path of laser light L1 (described later), and the heat insulating material D is configured not to block the laser light L1 by the opening Da.

  The laser light source 3 emits so-called pump light for optically pumping the alkali metal 10 in the glass cell 2 toward the glass cell 2. The laser light source 3 emits laser light L0 having a wavelength capable of pumping the alkali metal 10.

  On the optical path of the laser light L0 emitted from the laser light source 3, a lambda plate 7 that controls the polarization direction of the laser light L0 to become the laser light L is disposed. On the optical path of the laser beam L, a half mirror 8 is disposed that reflects the laser beam L toward the glass cell 2 as the laser beam L1 and transmits the laser beam L2 as it is and proceeds.

  Further, on the downstream side of the glass cell 2 on the optical path of the laser light L1, a mirror 9a that appropriately reflects the laser light L1 transmitted through the glass cell 2 and enters the detector 4 is disposed. On the other hand, on the optical path of the laser beam L2, mirrors 9b and 9c for appropriately reflecting the laser beam L2 and making it incident on the detector 4 are arranged.

  The detector 4 detects a detection signal related to the magnetic field of the magnetic field source P, and includes light receiving units 4a and 4b. The light receiving unit 4a receives laser light L1 as transmitted laser light that has passed through the glass cell 2. The light receiving unit 4b receives a laser beam L2 as a reference laser beam that does not pass through the glass cell 2. For example, photodiodes are used as the light receiving portions 4a and 4b. The detector 4 is connected to a controller 6, and its operation is controlled by the controller 6.

  The detector 4 configured in this manner detects a change in the light intensity of the laser light L1 before and after transmission through the glass cell 2 from the difference in light intensity between the received laser lights L1 and L2, and based on this change in light intensity, the magnetic field source P Measure the magnetic field strength. The detector 4 outputs the measured magnetic field strength to the controller 6 as a detection signal.

  The high-frequency power source 5 applies a high-frequency voltage to a pair of electrodes (application portions) 12 provided in the glass cell 2 and heats the glass cell 2 by dielectric heating. The high frequency power supply 5 is connected to a controller 6, and its operation is controlled by the controller 6.

  The controller 6 controls the optical pumping magnetometer 1 and includes, for example, a CPU, a ROM, a RAM, and the like. The controller 6 includes a control device (control unit) 6a and a storage device 6b. The control device 6a stores the detection signal input from the detector 4 in the storage device 6b. Further, the control device 6 a controls the operation of the detector 4 and the high frequency power supply 5.

  Here, as shown in FIG. 2, in the glass cell 2 of the present embodiment, the pair of side surfaces 21 and 22 that do not intersect with the optical path of the laser light L <b> 1 and that face each other are disposed on the first electrode as the electrode 12. 12a and a second electrode 12b are provided. That is, the electrodes 12a and 12b are provided on the outer surface of the glass cell 2 so as not to block the laser light L1.

  The first electrode 12 a is electrically fixed to the side surface 21 so as to spread over the entire side surface (one outer surface) 21. The second electrode 12 b is electrically fixed to the side surface 22 so as to spread over the entire side surface (other outer surface) 22. The first and second electrodes 12a and 12b are made of a nonmagnetic metal and have a thin plate shape. The first and second electrodes 12a and 12b are electrically connected to the high frequency power source 5 (see FIG. 1) via lead wires 5a made of platinum or the like.

  Further, in the glass cell 2, the side surface 23 other than the side surfaces 21 and 22 does not intersect the optical path of the laser beam L1, and protrudes outward as a sealing mark formed when the internal space 2a is sealed at the time of manufacture. A sealing cut 24 is provided. That is, the electrodes 12 a and 12 b are provided on the side surfaces 21 and 22 where the sealing cut 24 does not exist.

  Returning to FIG. 1, the optical pumping magnetometer 1 of this embodiment includes a magnetic shield 13 and a Helmholtz coil 14 for shielding an external magnetic field during measurement. The magnetic shield 13 is a plate-like member that covers the magnetic field source P, the glass cell 2, and the heat insulating material D, and is formed of, for example, permalloy having a high magnetic permeability. In the magnetic shield 13, an opening 13a is formed on the optical path of the laser beam L1. The opening 13a prevents the laser beam L1 from being blocked by the magnetic shield 13.

  The Helmholtz coil 14 is provided so as to cover the magnetic field source P, the glass cell 2, the heat insulating material D, and the magnetic shield 13, that is, these are arranged in the coil. The Helmholtz coil 14 is connected to a Helmholtz coil power source 15 such as a DC power source.

  In the optical pumping magnetometer 1 configured as described above, the alkali metal 10 in the glass cell 2 exists in a solid or semi-solid state in the initial state. When measuring the magnetic field of the magnetic field source P, first, the high frequency power supply 5 is operated by the control device 6a, the high frequency voltage is applied to the first and second electrodes 12a, b, and the glass cell 2 itself becomes a heat source. Then, the glass cell 2 is heated (heated up) to a predetermined temperature by dielectric heating. Thereby, the alkali metal 10 in the glass cell 2 is vaporized.

  Subsequently, the control device 6a controls the high-frequency power source 5 to perform on-off control of the high-frequency voltage applied to the first and second electrodes 12a, 12b, for example, temperature control so that the glass cell 2 becomes constant at a predetermined temperature. In this state, the laser beam L0 is emitted from the laser beam source 3. The emitted laser beam L0 is converted into a laser beam L by the lambda plate 7, and then branched by the half mirror 8 into a laser beam L1 reflected toward the glass cell 2 and a laser beam L2 that travels as it is.

  The laser beam L1 enters the glass cell 2 and is transmitted therethrough. At this time, the electrons of the alkali metal 10 in the glass cell 2 are selectively excited by the laser light L1, and the direction of the electron spin is aligned (optical pumping). At the same time, since the aligned electron spin directions are shifted due to the influence of the magnetic field source P, the laser light L1 is light-absorbed according to the magnitude of the magnetic field of the magnetic field source P when the shift is to be restored. .

  The laser beam L1 transmitted through the glass cell 2 is reflected by the mirror 9a and is incident on the light receiving unit 4a of the detector 4. On the other hand, the laser beam L2 is reflected by the mirrors 9b and 9c and directly incident on the light receiving unit 4b of the detector 4.

  Subsequently, the operation of the high frequency power source 5 is stopped by the control device 6a, and the high frequency voltage is not applied to the first and second electrodes 12a, 12b. In this state, the detector 4 measures the light intensity of the laser light L2 that has passed through the glass cell 2, and measures the light intensity of the laser light L2.

  Subsequently, the light intensity difference between the laser beams L1 and L2 is calculated by the detector 4, thereby detecting a change in the light intensity of the laser beam L1 before and after transmission through the glass cell 2 to thereby determine the magnetic field intensity of the magnetic field source P. taking measurement. Then, the measured magnetic field intensity of the magnetic field source P is transmitted as a detection signal to the storage device 6b via the control device 6a and stored in the storage device 6b.

  In the present embodiment, before measuring the magnetic field of the magnetic field source P, the detector 4 may be calibrated (measurement calibration) in advance. Specifically, in a state where the alkali metal 10 is not vaporized before the high frequency power supply 5 is operated, the laser light L0 is emitted from the laser light source 3, and the light intensity difference between the laser lights L1 and L2 is calculated by the detector 4. The light intensity difference at this time may be calibrated to zero.

  As described above, in the optical pumping magnetometer 1 of the present embodiment, the glass cell 2 can be heated by dielectric heating by applying a high-frequency voltage to the glass cell 2 through the electrodes 12a and 12b from the high-frequency power source 5. That is, the glass cell 2 itself can be caused to generate heat by the dielectric loss of the glass cell 2, and the glass cell 2 can be uniformly heated. Therefore, the glass cell 2 can be easily and uniformly heated, and the sensitivity of the magnetic field measurement of the magnetic field source P can be easily increased.

  Further, in the optical pumping magnetometer 1, the cooling function required for the SQUID sensor can be eliminated, and the running cost can be reduced.

  In the present embodiment, as described above, the first electrode 12a and the second electrode 12b are electrically connected to the opposing side surfaces 21 and 22 of the glass cell 2 so as to spread over the entire area. Thereby, the dielectric loss of the glass cell 2 is uniformly generated in the whole glass cell 2, and it becomes possible to heat the glass cell 2 more accurately and uniformly.

  In the present embodiment, when the detection signal is detected by the detector 4 and stored in the storage device 6b by the control device 6a, the application of the high-frequency voltage by the high-frequency power source 5 is stopped. Thereby, it is possible to suppress the adverse effect due to the magnetic field (for example, the magnetic field generated from the lead wire 5a) generated by the operation of the high-frequency power supply 5 from reaching the magnetic field measurement of the magnetic field source P.

  Moreover, in this embodiment, as above-mentioned, the magnetic field source P and the glass cell 2 are covered with the magnetic shield 13 and the Helmholtz coil 14, and the magnetic noise from the outside is interrupted | blocked (cancelled). Therefore, it is possible to suppress the adverse effect of the external magnetic field from affecting the magnetic field measurement of the magnetic field source P.

  Further, in the present embodiment, as described above, the laser beam L formed by controlling the polarization direction of the laser beam L0 is changed into the laser beam L1 whose light intensity changes and the laser beam L2 as the reference laser beam. And is detected by the detection unit 4. The detector 4 detects a change in the light intensity of the laser light L1 before and after transmission through the glass cell 2 from the difference in light intensity between the laser lights L1 and L2. Therefore, even when the light intensity of the emitted laser light L0 fluctuates slightly without being stabilized, the light intensity change of the laser light L1 can be detected so as to follow the fine fluctuation, and the magnetic field measurement can be performed with high accuracy. It can be carried out.

  Moreover, in this embodiment, since the heat insulating material D is installed in the circumference | surroundings of the glass cell 2, it becomes possible to interrupt | block so that the heat of the glass cell 2 may not escape to the exterior.

  The preferred embodiments of the present invention have been described above. However, the present invention is not limited to the above-described embodiments. The present invention is modified without departing from the gist described in each claim or applied to other embodiments. It may be a thing.

  For example, the optical pumping magnetometer 1 of the above embodiment may directly apply a conductive material such as graphite to the outer surface of the glass cell 2 and apply a high frequency voltage from the high frequency power source 5 using the conductive material as an application unit. In addition, the application unit may have light transparency. For example, as shown in FIG. 3, instead of the electrodes 12a and 12b (see FIG. 2), a transparent conductive film such as ITO having light transparency. 12a 'and 12b' may be provided.

  Moreover, in the said embodiment, the laser beam L1 which permeate | transmitted the glass cell 2 is received as a permeation | transmission laser beam, and the intensity | strength of the magnetic field of the magnetic field source P was measured based on the light intensity change of the laser beam L before and behind permeation | transmission of the glass cell 2. However, in the present invention, it is not limited to the magnetic field measurement of the magnetic field source P by such light absorption, and for example, the magnetic field measurement of the magnetic field source P using Faraday rotation can be adopted.

  Specifically, for example, as shown in FIG. 3, a so-called probe light is used so as to intersect the laser light L1 in the glass cell 2 by another laser light source (not shown) different from the laser light source 3. A certain linearly polarized laser beam L ′ is separately irradiated toward the glass cell 2. At the same time, the linearly polarized laser beam L transmitted through the glass cell 2 is received as a transmitted laser beam. Then, a detection signal as the intensity of the magnetic field of the magnetic field source P may be detected based on a change in polarization state (change in polarization direction) of the linearly polarized laser beam L ′ before and after transmission through the glass cell 2. This is because when the linearly polarized laser beam L ′ is incident on the alkali metal 10 optically pumped with the laser beam L1, the polarization direction of the transmitted linearly polarized laser beam L ′ is changed to a magnetic field due to the magneto-optical effect due to Faraday rotation. This is because it is rotated by an angle corresponding to the size of.

  In addition, when irradiating the linearly polarized laser beam L ′ to the glass cell 2 in this way, it is preferable that the optical path of the linearly polarized laser beam L ′ does not pass through the sealing cut 24. Thereby, it is possible to suppress an adverse effect on the polarization state of the linearly polarized laser beam L ′ due to the presence of the sealing cut 24. Further, in this case, it is preferable that the application unit has light transmittance. For example, as shown in the drawing, transparent conductive films 12a 'and 12b' such as ITO having light transmission property are preferably used as the application unit. Thereby, even when an application unit is provided on the optical path of the linearly polarized laser beam L ′, it is possible to suppress deterioration in accuracy of magnetic field measurement by the application unit.

  In the above embodiment, the electrodes 12a and 12b are fixed to the side surfaces 21 and 22, respectively. However, the electrodes 12a and 12b may be simply placed on the side surfaces 21 and 22, for example. , 22 may be electrically connected. In the above embodiment, when there are a plurality of magnetic field sources P, the glass cell 2 may be irradiated with a plurality of laser beams L1 so as to be close to the plurality of magnetic field sources P.

  Moreover, the structure which concerns on the optical path of laser beam L, L1, L2 is not limited above, For example, according to arrangement | positioning relationship with the other site | part of the optical pumping magnetometer 1, it can comprise suitably. Moreover, although the said glass cell 2 is exhibiting the cube external shape, it is not limited to this, The polyhedral outer diameter and the spherical body external shape may be exhibited.

  Moreover, in the said embodiment, although electrode 12a, 12b is provided as an application part in the side surfaces 21 and 22 opposing each other, you may provide in a part of side surfaces 21 and 22. FIG. When the cells are suitably heated evenly, it is only necessary that the application unit be provided on the pair of outer surfaces that are the farthest from each other among the plurality of outer surfaces of the cell. It only has to be. In the above, the laser light source 3 and the lambda plate 7 constitute a laser light irradiation part, and the magnetic shield 13 and the Helmholtz coil 14 constitute an external magnetic field shielding part. For example, in order to achieve the above-described effects, the laser light L is preferably a circularly polarized light or a linearly polarized laser light.

  According to the present invention, the cells can be easily and uniformly heated.

  DESCRIPTION OF SYMBOLS 1 ... Optical pumping magnetometer, 2 ... Glass cell (cell), 3 ... Laser light source (laser light irradiation part), 4 ... Detector (detection part), 5 ... High frequency power supply (high frequency voltage application part), 6a ... Control Device (control unit), 7 ... Lambda plate (laser light irradiation unit), 10 ... Alkali metal, 12a ... First electrode (application unit), 12b ... Second electrode (application unit), 12a ', 12b' ... Transparent conductivity Membrane (applying portion), 13 ... magnetic shield (external magnetic field shielding portion), 14 ... helmholtz coil (external magnetic field shielding portion), 21 ... side surface (one outer surface), 22 ... side surface (other outer surface), L, L2... Laser light, L1... Laser light (transmission laser light), L ′... Linearly polarized laser light (transmission laser light), P.

Claims (8)

An optical pumping magnetometer that measures the magnetic field to be measured using optical pumping,
A non-magnetic cell having at least an alkali metal encapsulated and having light transmittance and heat resistance;
A laser beam irradiation unit configured to irradiate the cell with a laser beam and optically pump at least the alkali metal;
A detector that receives the transmitted laser light transmitted through the cell and detects a detection signal related to the magnetic field;
An optical pumping magnetometer, comprising: a high-frequency voltage application unit that applies a high-frequency voltage to a pair of application units provided in the cell and heats the cell by dielectric heating. The pair of application units includes
A first electrode electrically connected to one outer surface of the cell so as to extend across the entire outer surface;
2. The optical pumping magnetometer according to claim 1, further comprising: a second electrode electrically connected to another outer surface facing the one outer surface of the cell so as to extend across the other outer surface.   3. The optical pumping magnetometer according to claim 1, wherein the pair of application units has optical transparency. A control unit for controlling operations of the detection unit and the high-frequency voltage application unit;
The optical pumping magnetometer according to any one of claims 1 to 3, wherein when the detection unit detects the detection signal, the control unit stops the application of the high-frequency voltage by the high-frequency voltage application unit.   The optical pumping magnetometer according to any one of claims 1 to 4, further comprising an external magnetic field shielding unit that covers the measurement target and the cell so as to shield an external magnetic field. The detector is
Receiving the laser beam irradiated by the laser beam irradiation unit and transmitted through the cell as the transmitted laser beam, and detecting the detection signal based on a change in light intensity of the laser beam before and after transmission through the cell; or
A linearly polarized laser beam irradiated by another laser beam irradiation unit different from the laser beam irradiation unit and transmitted through the cell is received as the transmitted laser beam, and a polarization state of the linearly polarized laser beam before and after transmission through the cell The optical pumping magnetometer according to claim 1, wherein the detection signal is detected based on a change.   A magnetoencephalograph comprising the optically pumped magnetometer according to claim 1.   An MRI apparatus comprising the optical pumping magnetometer according to claim 1.

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